Abstract: An algorithm of minimum mean square error (MMSE) quadrature demodulation is studied and applied in ultrasonic blood flow imaging. At first, an iterative form of the MMSE is established to obtain the amplitudes and phases of the in-phase and quadrature components of the echo signals by comparing the desired signals and the received signals. Then, the autocorrelation method is used to estimate the blood flow velocity using the signals obtained from the MMSE. The simulation results show that, for sinusoidal modulation signals with Gaussian noise of different energy and SNRs from 0.5 dB to 10 dB, the mean SNR of the output signals with this algorithm is higher 15 dB and 4 dB than the SNR with Hilbert algorithm and I/Q demodulation algorithm, respectively The simulation results of the flow imaging show that the required amount of multiplication has been reduced to 18% and 9% compared with the above methods when they have the comparable performance on blood flow velocity estimation. Therefore, the application of the MMSE for ultrasonic blood flow imaging has certain significance for improving the estimation performance and reducing the computation amount.